1. Field of the Invention
The present invention relates to a cover film for a sublimation thermal transfer hard copy. The term "sublimation thermal transfer hard copy" is used in this specification to mean a hard copy of an image formed on an image receiving sheet by a sublimation thermal transfer method.
2. Related Background Art
Hitherto, an intense study has been made for developing methods for reproducing an image in the form of a hard copy similar to a photograph from electrical picture signals derived from, for example, a video camera, a still video camera, a television, a video disk and a photograph transmission system. Among these methods, a method called "sublimation thermal transfer (image) recording method" is now attracting attention.
Briefly, the sublimation thermal transfer recording method is a method in which electrical signals containing picture data are delivered to a thermal head having an array of electric heat generating elements arranged at a density of, for example, 4 to 16 dots (elements) per 1 mm, while an image-receiving sheet and an ink sheet superposed on the image-receiving sheet, the ink sheet being usually composed of a carrier sheet and a sublimation dyestuff layer on the carrier sheet, are moved together in contact with the head, so that the dyestuff is transferred in the form of a dot pattern from the ink sheet to the image-receiving sheet, whereby a hard copy is produced.
The thus obtained sublimation thermal transfer hard copies are usually stored in an office file, e.g., on a sheet of paper, in a notebook, a book, a scrap book, a card case, a letter case, and so forth, according to the uses of the hard copies.
When a sublimation thermal transfer copy is left for a long time with the image surface held in contact with an office file, the sublimation-type dyestuff sublimating from the image surface of the hard copy undesirably contaminates the portion of the office file contacting the hard copy.
In addition, discoloration or change in the color of the thus obtained hard copy image tends to be caused due to influence of heat, light or contact with gases, thus making it impossible to store the hard copy in good order.